Morphological features and evolution of Jupiter’s Polar Cyclones revealed from JunoCam and JIRAM.

Juno has been observing the evolution of Jupiter’s circumpolar cyclones (CPCs) with the visible-light camera, JunoCam, and the 2-5 µm infrared JIRAM camera/spectrometer, since orbit insertion.  The CPCs have distinctive cloud features, and unique characteristics that, at least in visible and infrared wavelengths, broadly classify into two morphological forms, “filled,” and “chaotic” as in Tabataba-Vakili et al. 2020. Here, we call the chaotic form “spiral” (Fig. 1).

Figure 1: Left. Junocam image of Jupiter’s north polar cyclones, a composite of perijoves (PJ) 35 and 36. Image processed by Gerald Eichstädt. 0º Longitude Sys. III to the right (near CPC #1). Right. JIRAM image of Jupiter’s north polar cyclones (right) showing longitudes and oriented the same as the JunoCam image.

As revealed by JunoCam and JIRAM, the filled CPCs typically appear with large, visibly bright, 5- μ cloud features on the periphery, similar in appearance to a circular saw blade.  Just inward of those, nearly uniform darker regions appear occasionally displaying small hole-like openings, appearing bright at 5 μm. These darker regions (e.g., Fig. 1 left, CPC #3 & Fig. 2 left) are probably a result of flat non-convective stratiform clouds. The overall appearance of the periphery and just inward is reminiscent of shear-like instability in the flow. Anticyclonic circulation has been witnessed in the center of several filled CPCs (see Eichstädt et al., this meeting). Lightning has also been observed by JunoCam in one of the blade-like cloud features during PJ 31, and we occasionally observe thin, bright curvilinear cloud features and clusters of bright clouds with shadows indicating vertical structure.

Figure 2: Filled CPC #1. JunoCam (left) and JIRAM (right) Lambert map-projections of CPC 1 from PJ 38.The spiral CPCs (Fig. 3), including the central, north polar cyclone have a different morphology than the filled cyclones, appearing as flocculent and tightly wrapped series of alternatively bright and dark spirals. Interestingly, CPC #2 has partially transformed from a chaotic morphology into a filled morphology, similar perhaps to how oval cyclones and barges in the low latitudes can sometimes transform into folded-filamentary cyclones (e.g., Clyde’s Spot; Hueso et al. 2022). Microwave radiometry (see Orton et al., this meeting) strongly suggests the north polar cyclone (NPC) is a third class of polar cyclone that morphologically appears as a spiral type but has a different vertical brightness temperature structure than possessed by any of the CPCs.

Figure 3: Lambert map-projections. Spiral CPC #2, PJ 38. JunoCam (left) and JIRAM (center). Spiral CPC #8, PJ 52 Junocam (right). The JunoCam (JIRAM) bright (dark) core is not always present in a spiral CPC as evidenced by CPC #8 in the right panel.  However, the tightly wrapped spiral arms are present outside the core and are distinctly different than the stratiform cloud deck and “bladed” cloud features of filled CPCs.

We discuss the morphology, cloud areal coverage, and evolution of each of the CPCs and NPC as revealed by JunoCam and JIRAM throughout the course of the mission thus far. This work is an attempt to document the cloud-top structure of Jupiter’s polar cyclones and their changes for future modeling attempts to replicate them in detail, which, in turn, may provide additional insight into their formation, evolution, and stability.